How to loop through 100 files to do solidify through script?
This should do the trick:

import bpy
import glob
import os


def process(stl_fname):
    print('Processing %r' % stl_fname)

    # Determine output filename.
    out_fname = stl_fname.replace('.stl', '-solid.stl')
    if out_fname == stl_fname:
        raise ValueError('stl_fname should end in .stl')

    # Perform the processing.
    bpy.ops.import_mesh.stl(filepath=stl_fname)
    bpy.ops.object.modifier_add(type='SOLIDIFY')
    bpy.context.object.modifiers["Solidify"].thickness = 0.03
    print('Writing to %r' % out_fname)
    bpy.ops.export_mesh.stl(filepath=out_fname)
    bpy.ops.object.delete()

# Start with nothing in the scene
bpy.ops.object.select_all(action='SELECT')
bpy.ops.object.delete()

# Process *.stl
here = bpy.path.abspath('//')
for stl_fname in glob.glob(os.path.join(here, '*.stl')):
    if '-solid.stl' in stl_fname:
        print('Skipping %r' % stl_fname)
        continue

    process(stl_fname)


The code processes all STL files that are in the same directory as the blend file it is run from.